Photoplethysmography, or “PPG”, is an optical technique for detecting blood volume changes in a tissue. In this technique, one or more emitters are used to direct light at a tissue and one or more detectors are used to detect the light that is transmitted through the tissue (“transmissive PPG”) or reflected by the tissue (“reflectance PPG”). The volume of blood, or perfusion, of the tissue affects the amount of light that is transmitted or reflected. Thus, the PPG signal may vary with changes in the perfusion of the tissue.
The blood volume in a tissue changes with each heartbeat, and so the PPG signal also varies with each heartbeat. Traditionally, this component of the PPG signal is referred to as the “AC component” component of the signal, and is also often referred to as the “pulsatile component.” Blood volume is also affected by other physiological processes in the body, including respiration, venous blood volume, sympathetic and parasympathetic tone and certain pathologies. The changes in the PPG signal due to these and other physiological processes, along with changes in the PPG signal due to noise caused by non-physiological processes such as ambient light and bodily movement, have traditionally been referred to collectively as the “DC component.”
The present inventors have recently extracted specific parameters from the DC component, leaving the AC component signal to be used for monitoring traditional pulse oximetry physiological parameters, such as blood oxygen saturation and heart rate. Traditional sites for monitoring PPG, such as fingers and toes, generally provide a relatively small PPG signal, and the quality of this signal may be negatively impacted by sympathetic innervation in these tissue sites. Thus, the DC component signal from traditional peripheral sites may not be of sufficient strength and quality to effectively separate out the signals from different physiological processes.
The nasal alar region has recently been shown by the inventors to provide a very large PPG signal relative to other sites of the body, including the fingers, toes and ears, and a relatively high quality signal due to its lack of sympathetic innervation. The improved PPG signal at the nasal alar site has allowed for a number of physiological parameters, including respiration rate, respiratory effort and venous capacitance to be effectively extracted from the DC signal. Examples of patents and applications that describe the use the nasal alar site to obtain PPG signals, as well as a description of parameters and physiological processes that may be extracted from such signals, include U.S. Pat. Nos. 6,909,912; 7,127,278; 7,024,235; 7,785,262; 7,887,502 and 8,161,971, the entire contents of each of which are incorporated herein by reference in their entirety.
Thus, new devices, systems and methods for PPG monitoring at the nasal alar site may be desirable. Furthermore, new devices, systems and methods for PPG monitoring generally may also be advantageous.